1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head, more specifically to the use of either transverse or longitudinal synthetic exchange biasing to stabilize, suppress side reading and reduce the magnetic track width (MRW) of such a sensor.
2. Description of the Related Art
Magnetic read heads whose sensors make use of the giant magnetoresistive effect (GMR) in the spin-valve configuration (SVMR) base their operation on the fact that magnetic fields produced by data stored in the medium being read cause the direction of the magnetization of one layer in the sensor (the free magnetic layer) to move relative to a fixed magnetization direction of another layer of the sensor (the fixed or pinned magnetic layer). Because the resistance of the sensor element is proportional to the cosine of the (varying) angle between these two magnetizations, a constant current (the sensing current) passing through the sensor produces a varying voltage across the sensor which is interpreted by associated electronic circuitry. The accuracy, linearity and stability required of a GMR sensor places stringent requirements on the magnetization of its fixed and free magnetic layers. The fixed layer, for example, has its magnetization “pinned” in a direction normal to the air bearing surface of the sensor (the transverse direction) by an adjacent magnetic layer (typically an antiferromagnetic layer) called the pinning layer. The free layer is typically magnetized in a direction along the width of the sensor and parallel to the air bearing surface (the longitudinal direction). Layers of hard magnetic material (permanent magnetic layers) or laminates of antiferromagnetic and soft magnetic materials are typically formed on each side of the sensor and oriented so that their magnetic field extends in the same direction as that of the free layer. These layers, called longitudinal bias layers, maintain the free layer as a single magnetic domain and also assist in linearizing the sensor response by keeping the free layer magnetization direction normal to that of the fixed layer when quiescent. Maintaining the free layer in a single domain state significantly reduces noise (Barkhausen noise) in the signal produced by thermodynamic variations in domain configurations. A magnetically stable spin-valve sensor using either hard magnetic biasing layers or ferromagnetic biasing layers is disclosed by Zhu et al. (U.S. Pat. No. 6,324,037 B1) and by Huai et al. (U.S. Pat. No. 6,222,707 B1).
The importance of longitudinal bias has led to various inventions designed to improve the material composition, structure, positioning and method of forming the magnetic layers that produce it. One form of the prior art provides for sensor structures in which the longitudinal bias layers are layers of hard magnetic material (permanent magnets) that abut the etched back ends of the active region of the sensor to produce what is called an abutted junction configuration. This arrangement fixes the domain structure of the free magnetic layer by magnetostatic coupling through direct edge-to-edge contact at the etched junction between the biasing layer and the exposed end of the layer being biased (the free layer). Another form of the present art employs patterned direct exchange bias. Unlike the magnetostatic coupling resulting from direct contact with a hard magnetic material that is used in the abutted junction, in exchange coupling the biasing layer is a layer of ferromagnetic material which overlays the layer being biased, but is separated from it by a thin coupling layer of conducting, but non-magnetic material. This non-magnetic gap separating the two layers produces exchange coupling between them, a situation in which it is energetically favorable for the biasing layer and the biased layer assume a certain relative direction of magnetization. Another form of exchange coupling involves a direct contact between the free ferromagnetic layer and an overlaying layer of antiferromagnetic material. Xiao et al. (U.S. Pat. No. 6,322,640 B1) disclose a method for forming a double, antiferromagnetically biased GMR sensor, using as the biasing material a magnetic material having two crystalline phases, one of which couples antiferromagnetically and the other of which does not. Fuke et al. (U.S. Pat. No. 6,313,973 B1) provides an exchange coupled configuration comprising a coupling film, an antiferromagnetic film and a ferromagnetic film and wherein the coupling film has a particularly advantageous crystal structure.
As the area density of magnetization in magnetic recording media (eg. disks) continues to increase, significant reduction in the width of the active sensing region (trackwidth) of read-sensors becomes necessary. For trackwidths less than 0.2 microns (μm), the traditional abutted junction hard bias structure discussed above becomes unsuitable because the strong magnetostatic coupling at the junction surface actually pins the magnetization of the (very narrow) biased layer (the free layer), making it less responsive to the signal being read and, thereby, significantly reducing the sensor sensitivity.
Under very narrow trackwidth conditions, the exchange bias method becomes increasingly attractive, since the free layer is not reduced in size by the formation of an abutted junction, but extends continuously across the entire width of the sensor element. FIG. 1 is a schematic depiction of an abutted junction arrangement and FIG. 2 is an equally schematic depiction of a direct exchange coupled configuration. As can be seen, the trackwidth in the abutted junction is made narrow by physically etching away both ends of the sensor, whereas in the exchange coupled sensor, the trackwidth is defined by placement of the conductive leads and bias layers while the sensor element retains its full width.
The direct exchange biasing-also has disadvantages when used in a very narrow trackwidth configuration because of the weakness of the pinning field, which is found to be, typically, approximately 250 Oe. The present invention will address this weak pinning field problem while retaining the advantages of exchange biasing by providing a new exchange biased configuration, synthetic exchange biasing. In this configuration, the biasing layer is exchange coupled to the free layer by antiferromagnetic exchange coupling, in which the ferromagnetic biasing layer and the ferromagnetic free layer are coupled by a non-magnetic layer to form a configuration in which the two layers have antiparallel magnetizations (a synthetic antiferromagnetic layer). A stronger pinning field, typically exceeding 700 Oe, can be obtained using the synthetic exchange biasing method. More advantageously, an effective magnetic trackwidth of 0.15 μm can be obtained with a physical track width of 0.1 μm by using such a configuration by reducing the level of side reading (sensor response generated by signals originating outside of the magnetic trackwidth region) which is produced by the portion of the free layer that is beneath the biasing layer and conduction leads. The invention provides such a novel synthetic exchange biased sensor in two configurations, longitudinal and transverse, each of which is shown to have particular advantages both in its operation and its formation.